For reasons of miniaturization of devices and also for reasons of economy, the demand for very small, but nevertheless high-power loudspeakers of external diameters of 50 mm, 30 mm, 15 mm, 10 mm down to 6 mm is continually rising. Synthetic resin foils are mainly used for the manufacture of diaphragms of loudspeakers of this size, the material used being in particular polycarbonate or alternatively polyarylate in material thicknesses from approximately 8 μm up to 150 μm. Corrugations are provided for these diaphragms which stabilize the diaphragms mechanically, because the corrugations counteract bending of the diaphragm, while at the same time they support the diaphragm in achieving a maximum stroke.
For this purpose, for example, corrugations leading from the inside to the outside are known which are provided in approximately radial directions and which are straight or curved in shape. Usually, the corrugations extend so as to radiate away from the retaining part of the diaphragm provided for retaining the moving coil to the fastening part of the diaphragm provided for fastening the diaphragm, and are evenly distributed over the diaphragm, for example at equal angular distances. The cross-section of the corrugations is then, for example, V-shaped, the cross-section in the center of each corrugation often being greater than at the ends of the corrugation.
The radiating arrangement of the corrugations provides the necessary longitudinal compensation within the diaphragm, when the latter is made to move during operation of a loudspeaker. Usually, in fact, a diaphragm comprises a vaulty sound-generating surface, which corresponds approximately to a toroidal surface having a circular cross-section. As a result, a point lying on the sound-generating surface of the diaphragm moves partly in radial direction during a movement of this diaphragm, which leads to a change in circumference of the circular arc passing through said point. The longitudinal compensation necessary for a maximum stroke of the diaphragm, and thus for a maximum acoustic strength, is made possible by the corrugations extending in radial directions, which corrugations narrow or widen to a greater or lesser degree each time.
It should be noted here that corrugations in a radiating arrangement are usually also provided in the vaulty sound-generating surface of the diaphragm in the case of elongated loudspeakers with an elongated diaphragm, so as to achieve the said longitudinal compensation upon a movement of the sound-generating surface of the diaphragm. Such a compensation, however, is not necessary in those portions of the diaphragm in which the edges or longitudinal sides of the retaining part and of the fastening part run mutually parallel, because points lying on the diaphragm do not move over a toroidal surface but over a cylindrical surface upon a movement of the diaphragm, which obviously does not lead to a change in distance between the points. Corrugations perpendicular to the cylinder axis, moreover, would hamper rather than facilitate the rolling movement of the cylindrical surface resulting from a movement of the diaphragm.
In contrast to circular loudspeakers, the operation of elongated loudspeakers leads to problems because of the asymmetry explained above, owing to the asymmetrical mechanical load on the moving coil connected to the retaining part of the diaphragm. During vibration of the diaphragm, in fact, radial forces of different values act on the moving coil, and the diaphragm may be warped, said forces leading to an unfavorable mechanical load on the moving coil. The frequently used high-pressure deep-drawing process, in which a synthetic resin foil is heated up to the glass transition temperature of approximately 220° C. and is subsequently pressed onto a die under a pressure of 20 bar to 25 bar, also leads to an intensification of the problem. The difference in longitudinal stretching of the diaphragm tends to make the thickness of the diaphragm in the at least one narrow region greater than in the at least one wide region, which unfortunately contributes to an undesirably high stiffness in the at least one narrow region.
Life problems occur in particular in the case of self-supporting moving coils, i.e. moving coils in which the individual turns are connected by means of only small quantities of adhesive. The individual turns are interconnected in the manufacture of the moving coil in that a basic coil manufactured from a coated coil wire is heated, which leads to liquidizing and subsequent adhesion of the lacquer-type layer provided on the coil wire. The moving coils thus manufactured, however, can be exposed to weak mechanical loads only because of the small adhesion surface areas between the individual turns.
Elongated loudspeakers with elongated diaphragms are accordingly provided with comparatively small cylindrical moving coils in relation to the smallest dimension of the elongated diaphragm, in order to keep disadvantageous mechanical influences on the moving coil as small as possible, because this makes the difference between the various radial forces comparatively small, and the mechanical load on the moving coil can thus be kept within acceptable limits. A small coil diameter of the moving coil, however, is not in accordance with the requirement of a magnet system that is as large as possible, which is necessary for achieving a maximum acoustic output.
In the prior art, therefore, elongated, preferably oval moving coils are used for elongated loudspeakers so as to avoid the problems described above. The constant distance between the retaining part for retaining the moving coil and the fastening part for fastening the diaphragm to a housing of the loudspeaker rendered possible thereby achieves that the mechanical forces exerted by the diaphragm on the moving coil during a movement of the diaphragm takes place centrally symmetrically, and is accordingly practically equally great all round. Oval moving coils, however, are much more difficult to manufacture than circularly cylindrical moving coils and require a high-precision mounting so as to achieve an exact angular relative position of the moving coil with respect to the magnet system of the loudspeaker. This is necessary because the air gap in the magnet system for accommodating the moving coil is made as narrow as possible so as to achieve as high as possible an efficiency, and a small angular misalignment of the moving coil in the air gap already can lead to a malfunction of the loudspeaker and to damage or even destruction of the moving coil. The effort required for avoiding the above risks renders the manufacture of such loudspeakers with such diaphragms much more expensive, which leads to competitive disadvantages because of the price pressure in particular in the field of small consumer electronics devices.
Different distances between the retaining part for the moving coil and the fastening part for fastening the diaphragm are also present in an elongated loudspeaker with an elongated diaphragm and with a cylindrical moving coil. It is known to choose the bulge of the diaphragm in the region of the two narrow regions present in this case to be greater than that in the region of the two wide regions present in this case in order to render it possible to obtain the same axial stroke all round for the retaining part provided for retaining the moving coil. The rolling movement occurring here upon a movement of the diaphragm and discussed above is accordingly more intensive in the two narrow regions than in the two wide regions. For this reason, and also because the torque causing the rolling movement is smaller in the two narrow regions because of the shorter lever arm than in the two wide regions, the diaphragm is deformed with greater difficulty in the two narrow regions than in the two wide regions. This leads to warping of the diaphragm and accordingly to warping of the basically planar formed retaining part as well as to radial deformations of the retaining part.